Suspend Validated.fx implementation

[aballano]

Done in this PR

• Add suspend fx for Validated
• Add Validated.fromNullable
• Fix an issue on Validated docs for error accumulation
• Rework Validated docs to include an fx example

Details

As per #116 this PR adds fx to validated, so one simple example can be found in the docs of validated itself, where we have the following code:

data class Config(val map: Map<String, String>) {
 fun <A> parse(read: Read<A>, key: String): Validated<ConfigError, A> {
  val v = Option.fromNullable(map[key])
  return when (v) {
   is Some ->
    when (val s = read.read(v.t)) {
     is Some -> s.t.valid()
     is None -> ConfigError.ParseConfig(key).invalid()
    }
   is None -> Validated.Invalid(ConfigError.MissingConfig(key))
  }
 }
}

So, instead of dealing with option, I added a Validated.fromNullable to match fromOption already present and simply deal with validates in an fx block:

data class Config(val map: Map<String, String>) {
  suspend fun <A> parse(read: Read<A>, key: String) = Validated.fx<ConfigError, A> {
    val value = Validated.fromNullable(map[key]) {
      ConfigError.MissingConfig(key)
    }.bind()

    val readVal = Validated.fromOption(read.read(value)) {
      ConfigError.ParseConfig(key)
    }.bind()

    readVal
  }
}

The only difference is that we force parse to be suspended because of this version of fx being suspended (shall we wait for #124?).

[pakoito]

Instead of Throw, we could return an Invalid with it.

[aballano]

But that's the impossible case, does it really matter? Actually I was looking for if there was an operator that does this already and doesn't leave the burden of writing this to the dev but couldn't see any

[pakoito]

Isn't it exhaustive then?

[aballano]

It cannot be because we're checking 2 elements

[pakoito]

we need pattern matching weep. I'd still use Invalid for the sake of not promoting exceptions. Not a blocker tho.

[nomisRev]

Yes, you can use Applicative for this.

Validated.applicative(NonEmptyList.semigroup<String>()).tupledN(
  1.valid(),
  "2".invalidNel(),
  "3".invalidNel()
) // Validated.Invalid(NonEmptyList("2", "3"))

You can always make these exhaustive by nesting when, which after a lot of back and forth have gone back to...

[aballano]

Right, I just realized that we're in core-data and we don't have access to the extensions :/

What I did is the following:

• Added a convenient nelApplicative function in Validated extensions
• Suggest to use this approach instead of the custom function in docs, but without changing the examples code due to the import problem

[pakoito]

It's just meh in general. It could be possible to rewrite it as two nested folds and some manual formatting.

[nomisRev]

I'm not a fan of showing parallelValidate at all.. It promotes/shows code that no-one should ever write using Arrow.

We have a bunch of helpers in the docs module, any way we can provide instances there to make the desired snippets compile?

[aballano]

The only reason I like it is for the explanation vs a Validated Nel, which is also explained in there. I changed it to try to make it clear that this is just for the shake of the example

[nomisRev]

Looks great!
We should refactor those snippets to use mapN or tupledN instead instead of parallelValidate before we merge.

[aballano]

@pakoito Would you take a second look at the docs when you have time, please? 🙏😁

[pakoito]

Neat

[pakoito]

Great! Good use of both APIs

[pakoito]

I was being extra nitpicky on the review. Nothing's a blocker really.

[aballano]

@nomisRev If you have any more comments later please let me know so I can open another PR :)